1. Field of the Invention
The present invention relates to a nickel positive electrode having improved characteristics for use in alkaline storage batteries using the positive electrodes of nickel oxide and the negative electrodes of a hydrogen storage alloy capable of electrochemically absorbing and desorbing hydrogen, cadmium, or zinc, and to a nickel-hydrogen storage battery having improved characteristics.
2. Description of the Related Art
Recently, there has been an intensive need for high energy density secondary batteries associated with portable appliances which are made much more valuable, compact and lighter in weight. Similarly, such new high energy density secondary cells are also vigorously sought for power supplies for electric automobiles. To cope with these needs, in the technical field of nickel-cadmium cell (referred to as Ni/Cd cell hereunder), there have been developed a Ni/Cd cell having a higher capacity using conventional sintered nickel positive electrodes and further a much higher energy density Ni/Cd cell having a capacity of 30 to 60% greater than that of the former using foamed metal type nickel positive electrodes. Moreover, there has been developed a nickel-hydrogen storage battery with negative electrodes of a hydrogen storage alloy having a higher capacity than those of Ni/Cd cells, i.e., a capacity two times or more as great as those of Ni/Cd cells using sintered nickel positive electrodes. These high capacity alkaline storage batteries include positive electrodes composed of a sintered porous nickel body, a bulk foamed porous nickel body, or a porous fibrous nickel body of a high porosity (90% or more) which are filled with nickel hydroxide powder to a high density in order to achieve an improvement in energy density of the positive electrodes. Consequently, modern sintered nickel positive electrodes have an improved energy density of 450 to 500 mAh/cm.sup.3, and foamed metal type nickel positive electrodes have an energy density of 550 to 650 mAh/cm.sup.3, as opposed to conventional sintered nickel positive electrodes having an energy density of 400 to 500 mAh/cm.sup.3. Those positive electrodes composed of a sintered porous nickel body, a foamed porous nickel body, or a porous fibrous nickel body which are filled with nickel hydroxide powder to a high density have a problem that their energy density can be maintained at higher levels about at normal temperature, but reduced at high temperature atmosphere. Therefore, it is difficult to take advantages of the characteristics of high energy density in a wider temperature range. This is attributed to the fact that a charge reaction accompanied by a conversion from nickel hydroxide to nickel oxyhydroxide and an oxygen evolving reaction are liable to simultaneously occur under a high temperature atmosphere. That is, an oxygen evolving overvoltage at positive electrodes is reduced leading to an insufficient charge reaction of the conversion of nickel hydroxide to nickel oxyhydroxide with a reduced rate of utilization of the nickel hydroxide. In order to overcome this problem, the following methods have been proposed:
(1) a method where cadmium oxide powder or cadmium hydroxide powder is added into positive electrodes, and
(2) a method where cadmium oxide is incorporated into the nickel hydroxide powder (Japanese Patent KOKAI (Laid-open) No. 61-104565) .
In the conventionally proposed methods (1) and (2) as above, the presence of cadmium oxide inside or with the nickel hydroxide powder improves the rate of utilization of the nickel hydroxide under a high temperature atmosphere. However, even when the addition of cadmium oxide is made, the rate of utilization of the nickel hydroxide under a high temperature atmosphere is only on the order of 80%. In order to improve the rate of utilization of the nickel hydroxide under a high temperature atmosphere, the amount of cadmium oxide to be added into inside the nickel hydroxide or into the nickel positive electrodes must be increased. However, the increase of the amount of cadmium oxide to be added causes a problem of adversely diminishing the rate of utilization of the nickel hydroxide about at normal temperature though the rate of utilization of the nickel hydroxide under a high temperature atmosphere may be improved up to about 90%.
In recent years, attention has been directed to a nickel-hydrogen storage battery containing no cadmium, heavy metal, from the point of environmental hazards. The removal of cadmium oxide from the nickel hydroxide or the positive electrodes, however, produces problems that the rate of utilization of the nickel hydroxide under a high temperature atmosphere is reduced to on the order to 50 to 60% and that when a sealed nickel-hydrogen storage battery with a positive electrode capacity controlled is arranged under less electrolyte conditions, the cycle life is shortened. The present invention is to overcome these problems by providing a nickel positive electrode for use in alkaline storage batteries having an excellent rate of utilization of the nickel hydroxide in an atmosphere in a wide range of temperature. Moreover, an object of the present invention is to provide a nickel-hydrogen storage battery being excellent in the cell capacity and in the cycle life in an atmosphere in a wide range of temperature.